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#### Heat Sink Temperature Calculator

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Power components have a maximum junction temperature, which must not be exceeded to prevent damage to the device. Devices are encapsulated in packages which have different levels of thermal resistance.  When designing power electronics, the heat dissipation of the device, coupled with any heat sinks, as well as the maximum power dissipated by the device, must be analyzed to insure that the device operates within allowable specified limits.

This calculator and tutorial computes the junction temperature of a power electronics device given the maximum ambient temperature, and the heat sink thermal resistance. The calculator will let you know if the maximum junction temperature could be exceeded for a given power, and likewise will tell you the maximum power that the device can handle for a given maximum power.

For automotive applications, use 80°C for the ambient temperature.  For maximum junction temperature, 150°C is a common value, but check the part's data sheet to be sure.  The junction to case thermal resistance varies by package, see the table below for common values. For the thermal resistance 1 field this could be the case to ambient temperature thermal resistance if there is no heat sink, or if a heat sink is used, it's the thermal resistance of the heat sink.  The second thermal resistance field is not typically used.

When choosing heat sinks, you want the smallest thermal resistance possible, which means that the heat will be more easily dissipated.

For surface mount (SMT) parts, where the PCB copper is used as a heat sink, for 1 ounce copper the heat dissipation asymptotically approaches 1 square inch, in other words, having a PCB heat sink greater than 1 inch doesn't do you any good. There are some tricks that can help, such as placing vias, into the pad, so that heat is transferred to the bottom layer as well. It's also possible to use surface mount heat sinks.

 Maximum Ambient Temperature (°C) Maximum Junction Temperature (°C) Thermal Resistance - Junction to Case (°C/Watt) Thermal Resistance 1 (°C/Watt) Thermal Resistance 2 (Optional) (°C/Watt) Results Junction Temperature   (°C) Power   (Watt)

#### Typical values of Thermal Resistance for Various Electronics Packages

 Package Junction to Case (°C/Watt) Junction to Air (°C/Watt) TO-3 5 60 TO-39 12 140 TO-220 3 62.5 TO-220FB 3 50 TO-223 30.6 53 TO-252 5 92 TO-263 23.5 50 D2PAK 4 35

#### Thermal Resistance for PCB Copper

 Heat Sink Thermal resistance (°C/Watt) 1 sq inch of 1 ounce PCB copper 43 .5 sq inch of 1 ounce PCB copper 50 .3 sq inch of 1 ounce PCB copper 56 Aavid Thermalloy, SMT heat sink:  PN:573400D00010 14

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